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Ok, here's the result of about an hour of fiddling with the cell adjustment screws last night.
There is a mismatch of about 1mm between the mechanical and the optical axis. This, I think, is due to a number of errors such as focuser tilt, probably a bit of lens cell and rotator eccentricity, as well as cross-hair eccentricity.
The cross-hair reticule came from my junk collection and I never tested it.
Anyway I adjusted the the lens cell so that the position of the dots don't change relative to the cross-hair when I rotate the focuser. That should produce consistent images regardless of camera orientation. Otherwise if I cancel the offset in one particular position, I get twice as much error when change orientation by 180 degrees.
The two attached images were taken at 180 degree rotation of the focuser, with fixed, tripod mounted camera looking through the eyepiece of the collimator.
And yes, Andy, there is a slight misalignment of the two lens groups as well, but I would not worry about that, as it is very minimal
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